The present invention relates generally to ion implantation systems, and more particularly to ion sources for providing ribbon beams in ion implantation systems.
Ion implantation systems or ion implanters are widely used to dope semiconductors with impurities in integrated circuit manufacturing, as well as in the manufacture of flat panel displays. In such systems, an ion source ionizes a desired dopant element, which is extracted from the source in the form of an ion beam of desired energy. The ion beam is then directed at the surface of the workpiece, such as a semiconductor wafer, in order to implant the workpiece with the dopant element. The ions of the beam penetrate the surface of the workpiece to form a region of desired conductivity, such as in the fabrication of transistor devices in the wafer. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particles. A typical ion implanter includes an ion source for generating the ion beam, a beamline including a mass analysis magnet for mass resolving the ion beam, and a target chamber containing the semiconductor wafer or other substrate to be implanted by the ion beam, although flat panel display implanters typically do not include a mass analysis apparatus. For high energy implantation systems, an acceleration apparatus may be provided between the mass analysis magnet and the target chamber for accelerating the ions to high energies.
Conventional ion sources include a plasma confinement chamber having an inlet aperture for introducing a gas to be ionized into a plasma and an exit aperture opening through which the plasma is extracted to form the ion beam. One example of gas is phosphine. When phosphine is exposed to an energy source, such as energetic electrons or radio frequency (RF) energy, the phosphine can disassociate to form positively charged phosphorous (P+) ions for doping the workpiece and hydrogen ions. Typically, phosphine is introduced into the plasma confinement chamber and then exposed to the energy source to produce both phosphorous ions and hydrogen ions. The plasma comprises ions desirable for implantation into a workpiece, as well as undesirable ions which are a by-product of the dissociation and ionization processes. The phosphorous ions and the hydrogen ions are then extracted through the exit opening into the ion beam using an extraction apparatus including energized extraction electrodes. Examples of other typical dopant elements of which the source gas is comprised include phosphorous (P), arsenic (As), or Boron (B).
The dosage and energy of the implanted ions are varied according to the implantation desired for a given application. Ion dosage controls the concentration of implanted ions for a given semiconductor material. Typically, high current implanters are used for high dose implants, while medium current implanters are used for lower dosage applications. Ion energy is used to control junction depth in semiconductor devices, where the energy levels of the ions in the beam determine the degree of depth of the implanted ions. The continuing trend toward smaller and smaller semiconductor devices requires a beamline construction which serves to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy permits shallow implants. In addition, the continuing trend toward higher device complexity requires careful control over the uniformity of implantation beams being scanned across the workpiece.
The ionization process in the ion source is achieved by excitation of electrons, which then collide with ionizable materials within the ion source chamber. This excitation has previously been accomplished using heated cathodes or RF excitation antennas. A cathode is heated so as to emit electrons Which are then accelerated to sufficient energy for the ionization process, whereas an RF antenna generates electric fields that accelerate plasma electrons to sufficient energy for sustaining the ionization process. The antenna may be exposed within the plasma confinement chamber of the ion source, or may be located outside of the plasma chamber, separated by a dielectric window. The antenna is energized by an RF alternating current which induces a time varying magnetic field within the plasma confinement chamber. This magnetic field in turn induces an electric field in a region occupied by naturally occurring free electrons within the source chamber. These free electrons accelerate due to the induced electric field and collide with ionizable materials within the ion source chamber, resulting in plasma currents within the ion chamber, which are generally parallel to and opposite in direction to the electric currents in the antenna. Ions can then be extracted from the plasma chamber by one or more energizable extraction electrodes located proximate a small exit opening, so as to provide a small cross-section (relative to the size of the workpiece)ion beam.
In many ion implantation systems, a cylindrical ion beam is imparted onto a wafer target through mechanical and/or magnetic scanning, in order to provide the desired implantation thereof. Batch implanters provide for simultaneous implantation of several wafers, which are rotated through an implantation path in a controlled fashion. The ion beam is shaped according to the ion source extraction opening and subsequent shaping apparatus, such as the mass analyzer apparatus, resolving apertures, quadrupole magnets, and ion accelerators, by which a small cross-section ion beam (relative to the size of the implanted workpiece) is provided to the target wafer or wafers. The beam and/or the target are translated with respect to one another to effect a scanning of the workpiece. However, in order to reduce the complexity of such implantation systems, it is desirable to reduce the scanning mechanisms, and to provide for elongated ribbon-shaped ion beams. For a ribbon beam of sufficient longitudinal length, a single mechanical scan may be employed to implant an entire wafer, without requiring additional mechanical or magnetic raster-type scanning devices. Accordingly, it is desirable to provide ribbon beam ion sources providing an elongated ion beam with a uniform longitudinal density profile for use in such implantation systems.
The present invention is directed to ion sources for ion implantation systems, by which an elongated or ribbon-shaped ion beam of uniform or controllable density may be provided for implanting a workpiece, such as a semiconductor wafer or flat-panel display. The invention provides ion sources in which a uniform plasma is provided within an elongated plasma confinement chamber, from which a ribbon-shaped ion beam is extracted through an elongated exit opening or extraction slit, having a relatively large aspect ratio. The elongated ribbon beam may then be used for implantation of semiconductor wafers in a single mechanical scan, thereby simplifying the implantation system. In one implementation, the invention can be employed to provide ribbon beams up to 400 mm in length, so as to facilitate single scan implantation of 300 mm semiconductor wafer workpieces.
In order to control the uniformity of the extracted ion beam, the invention advantageously provides coaxial RF excitation within a generally cylindrical source chamber to facilitate uniform generation of ionized plasma therein. Uniform plasma confinement within the plasma chamber is further enhanced through provision of circumferentially extending multi-cusp magnets providing azimuthal magnetic fields within the plasma chamber. An elongated exit opening or extraction slit is then provided in the plasma chamber for extraction using elongated energizable extraction electrodes to form a ribbon beam. The uniformity of the ions within the plasma chamber, in turn facilitates the provision of a uniform ribbon beam for uniformly implanting a wafer target having high feature density and small feature sizes. In addition, a thermal barrier, such as a cylindrical liner may be provided within the plasma chamber, which can rise to the plasma temperature, in order to mitigate condensation within the plasma chamber. This facilitates changeover from one implantation species to another without contamination from condensate common in prior RF excited ion (e.g., xe2x80x9ccold wallxe2x80x9d) sources.
One aspect of the invention provides an ion source, comprising a housing with a cylindrical plasma confinement chamber disposed along a longitudinal axis in which a plasma is generated, an antenna coaxially disposed in the plasma chamber along the axis, and an RF source for energizing the antenna. The housing comprises a cylindrical electrically conductive chamber wall extending longitudinally between first and second ends, with an elongated longitudinally extending exit opening through which an ion beam may be extracted from the plasma. The elongated exit opening may be of any longitudinal length, for example, such as about 400 mm, and may have a high aspect ratio to provide an elongated ribbon-shaped ion beam. The antenna comprises first and second terminals, with the first terminal being connected to the RF source and the second terminal being electrically connected to the chamber wall at the second end, where the chamber wall provides a return path for the RF source. A portion of the antenna between the first and second terminals extends longitudinally within the plasma confinement chamber along the axis for emitting energy into the plasma chamber.
The coaxial antenna thus facilitates uniform excitation of the plasma to provide a uniform ion source from which a ribbon beam may be extracted. The RF source has two outputs, including a first output connected to the first antenna terminal and a second output connected to the first end of the chamber wall. In this manner, the RF source, the antenna, and the chamber wall form a substantially coaxial electrical circuit to provide an alternating electric current in the exposed portion of the antenna for inducing an ionizing electric field within the plasma confinement chamber. Capacitors may be provided between the first RF source output and the antenna first terminal, and/or between the second end of the chamber wall and the second antenna terminal.
Another aspect of the invention provides an ion source for providing an ion beam in an ion implantation system, which comprises a housing defining a cylindrical plasma confinement chamber disposed along a longitudinal axis. The housing comprises a generally cylindrical electrically conductive chamber wall with an elongated longitudinally extending exit opening, and an antenna partially extending within the plasma confinement chamber for emitting energy therein. A plurality of magnets are provided, which are radially spaced from the axis within the plasma confinement chamber and longitudinally spaced from one another. Adjacent magnet pairs are of opposite magnetic polarity so as to create longitudinal magnetic fields near the chamber wall for confinement of plasma within the plasma confinement chamber. In one implementation, the magnets are permanent magnets individually extending circumferentially around a portion of the interior of the chamber wall between opposite sides of the exit opening.